The embodiment relates to a light emitting diode (LED) protection circuit, and more particularly, to an LED protection circuit using a one-shot vibrator.
When an LED backlight employing light emitting diodes (LEDs) uses a plurality of LEDs connected with each other in series, the LED backlight needs a driving circuit which can provide a constant current to the LEDs and a dimming circuit which can allow a user to arbitrarily control a luminance and the color temperature or to control a luminance of the LEDs to compensate for the temperature and the like. FIG. 1 shows a circuit diagram illustrating an LED driving circuit of the related art.
FIG. 1 is a circuit diagram illustrating one example of an LED driving circuit employing a conventional boost type DC-DC converter. As depicted in FIG. 1, in the LED driving circuit employing the conventional boost type DC-DC converter, an inductor L and a diode D are connected in series to a positive (+) terminal of a direct current source Vin, and a capacitor C and an LED array 11 are connected between the diode and a negative (−) terminal of the direct current source Vin in parallel to each other. A switch 12 and a voltage detection resistor Rs are connected in series between the connection node of the inductor L and the diode D and the negative (−) terminal of the direct current source Vin. A voltage value detected from the voltage detection resistor Rs is input to a PWM driver 13 and the PWM driver 13 controls an on-off duty ratio of the switch 12 according to a detected voltage value. As depicted in FIG. 1, a MOSFET may be used as the switch 2 by controlling a gate voltage of the MOSFET.
When the switch 12 is turned on, a current provided from the direct current source Vin flows through the inductor L and a switch S, such that energy is stored in the inductor L. When the switch 12 is turned off, a sum of the direct current source Vin and the stored energy of the inductor L is transferred to the LED array 11 through the diode D. At this time, a voltage transferred to the LED array 11 is smoothed by a smoothing capacitor C, such that a value of the voltage is equal to or greater than that of an input voltage Vin.
In such an LED driving circuit 10 employing a conventional boost type DC-DC converter, a luminance control of the LED array 11 is performed by controlling a resistance value of the resistor Rs to control a voltage value detected from the voltage detection resistor Rs. Although such a conventional luminance control can be performed by using a variable resistor as the voltage detection resistor Rs, since a wattage resistor must be used as the voltage detection resistor Rs due to a great LED resistance, there is a problem that it is not easy to control the voltage variation. Further, when the plurality of LEDs for generating various colors are used, different driving circuits are used for each LED color, so values of the voltage detection resistor Rs varies for each driving circuit, causing a bad influence upon a luminance adjustment, a color coordinate control, and uniformity.
Further, in the LED driving circuit 10 employing the boost type DC-DC converter, when the LED module 11 is open or a load is out of order, a current input to the LED module 11 is increased so that the module may be damaged. Therefore, an over-current protection circuit for an LED driving circuit has been essentially demanded.